7. Biodiversity Patterns in Relation to Climate
113
Sitka spruce (Picea sitchensis) is the principal species (along with western
hemlock) that defines the rainforest type throughout the entire coastal
Pacific forest zone. I will use Picea as the principal species to illustrate
how characteristics of history, ecology, and physiology influence the
genetic architecture of dominant species in this climatic zone. Picea is an
old genus, first appearing in the fossil record about 70 million years ago.
It has changed relatively little since first radiating from eastern Asia
(Wright, 1955). Most of the species of Picea developed as a result of
geographic isolation and can freely interhybridize . P. sitchensis is most
closely related to P. glauca and often hybridizes with it whenever their
geographic ranges overlap. It also freely interhybridizes with P. jezoensis,
P. omorica , and P. engelmannii (Wright, 1955). During the Pleistocene,
P. sitchensis maintained populations about 35 km south of San Francisco
Bay, on nunataks 1000m-1200m in elevation in coastal Alaska , and had
a major refugium south of Puget Sound (Daubenmire, 1968; Heusser,
1960; Hulten, 1937). P. sitchensis expanded to southern British Columbia
by 8600 years ago and to Southern Alaska by 8000 years ago, to Prince
William Sound 4000 ago, and to Kodiak 3000 years ago. It is still expanding its range westward of Kodiak , approximately 1.6 km per century
(Griggs, 1934; Heusser, 1960).
Picea sitchensis is extremely diverse throughout its range and appears
to be closely adapted to current climatic conditions (Falkenhagen, 1978;
Hamrick et al., 1992). P. sitchensis has a limited tolerance for moisture
stress, so is restricted to fog-influenced coastline in the southern part of
its range , and only reaches timberline in the perhumid and subpolar
rainforest types (Beadle & Jarvis, 1977; Watt, Nelson, & Jarvis, 1976). P.
sitchensis also has a relatively high salt tolerance, which explains in part
its dominance along beaches, particularly from British Columbia northward (Cordes , 1972). Geographic or microclimatic location of seed source
can be used to predict cone and seed morphology. Cone scale length:width
can be used to determine position on inland-coastal gradient (including
introgression with P. glauca) (Daubenmire, 1968; Falkenhagen, 1977,
1978). Introgression between P. sitchensis and P. glauca can also be
identified using isoenzyme techniques (Copes & Beckwith, 1977).
Genetically based differences in seedling growth appear to be related
to I timing of bud burst and the onset of dormancy , not photosynthetic
efficiency (Burley, 1965a, 1966). Bud burst is generally proportional to
latitude of the origin of the seed, as is nuclear volume (Burley, 1965b;
Roche 1969). Growth is also genetically controlled in relation to both
temperature regime and day length, with northern varieties growing and
reaching bud burst at lower temperatures (growing degree days). In
northern areas, southerly seed sources will produce faster-growing trees
but with increased risk of injury by frost (since bud burst is earlier and
dormancy is later).
113
Sitka spruce (Picea sitchensis) is the principal species (along with western
hemlock) that defines the rainforest type throughout the entire coastal
Pacific forest zone. I will use Picea as the principal species to illustrate
how characteristics of history, ecology, and physiology influence the
genetic architecture of dominant species in this climatic zone. Picea is an
old genus, first appearing in the fossil record about 70 million years ago.
It has changed relatively little since first radiating from eastern Asia
(Wright, 1955). Most of the species of Picea developed as a result of
geographic isolation and can freely interhybridize . P. sitchensis is most
closely related to P. glauca and often hybridizes with it whenever their
geographic ranges overlap. It also freely interhybridizes with P. jezoensis,
P. omorica , and P. engelmannii (Wright, 1955). During the Pleistocene,
P. sitchensis maintained populations about 35 km south of San Francisco
Bay, on nunataks 1000m-1200m in elevation in coastal Alaska , and had
a major refugium south of Puget Sound (Daubenmire, 1968; Heusser,
1960; Hulten, 1937). P. sitchensis expanded to southern British Columbia
by 8600 years ago and to Southern Alaska by 8000 years ago, to Prince
William Sound 4000 ago, and to Kodiak 3000 years ago. It is still expanding its range westward of Kodiak , approximately 1.6 km per century
(Griggs, 1934; Heusser, 1960).
Picea sitchensis is extremely diverse throughout its range and appears
to be closely adapted to current climatic conditions (Falkenhagen, 1978;
Hamrick et al., 1992). P. sitchensis has a limited tolerance for moisture
stress, so is restricted to fog-influenced coastline in the southern part of
its range , and only reaches timberline in the perhumid and subpolar
rainforest types (Beadle & Jarvis, 1977; Watt, Nelson, & Jarvis, 1976). P.
sitchensis also has a relatively high salt tolerance, which explains in part
its dominance along beaches, particularly from British Columbia northward (Cordes , 1972). Geographic or microclimatic location of seed source
can be used to predict cone and seed morphology. Cone scale length:width
can be used to determine position on inland-coastal gradient (including
introgression with P. glauca) (Daubenmire, 1968; Falkenhagen, 1977,
1978). Introgression between P. sitchensis and P. glauca can also be
identified using isoenzyme techniques (Copes & Beckwith, 1977).
Genetically based differences in seedling growth appear to be related
to I timing of bud burst and the onset of dormancy , not photosynthetic
efficiency (Burley, 1965a, 1966). Bud burst is generally proportional to
latitude of the origin of the seed, as is nuclear volume (Burley, 1965b;
Roche 1969). Growth is also genetically controlled in relation to both
temperature regime and day length, with northern varieties growing and
reaching bud burst at lower temperatures (growing degree days). In
northern areas, southerly seed sources will produce faster-growing trees
but with increased risk of injury by frost (since bud burst is earlier and
dormancy is later).